Scissor Advantage

Using units that are naturally more effective against the opposition in straightforward combat. The most traditional area of RTS games where prediction what your opponent will do will count.

The most known aspect is propably using different units doing different amounts of damage to different target types in order to deal more damage to the opponent than s/he can dish back. Proverbially if you see that your opponent is going to choose paper, send out scissors instead of rocks. To be interesting the different "X beats Y" relationsships are usually arranged in various Unit Counter Cycles

Other tactics in the cycle Rock Advantage , Paper Advantage

Downside is that the advantage is lost when opposition composition changes and a diverse army allows many errors in unit control.